1. Field of the Invention
The present invention relates to a magnetic flux meter using a DC driven type superconducting quantum interference device (hereinafter referred to as DC SQUID) and, more particularly, to a structure of a phase detecting circuit forming part of a flux locked loop (hereinafter referred to as F.L.L.) circuit device.
2. Description of the Prior Art
FIG. 4 is a block diagram showing on example of the structure of the conventional magnetic flux meter. A DC SQUID is provided for a sensor. A block enclosed by dotted lines, as designated at 21, is a phase detecting circuit. A modulated signal outputted from a DC SQUID 4 and amplified by an amplifier 5, is detected by a phase detector 22. The output signal of a square wave oscillator 6 or generating means is used as a signal for modulating the DC SQUID 4 and a reference signal for the phase detector 22. In order that the signal from the DC SQUID and the reference signal may have their phases coincided in the phase detector 22, the reference signal has its phase adjusted by a phase shifter 23.
Since the conventional DC SQUID magnetic flux meter is constructed as described above, an optimum output signal cannot be achieved without the phase adjustment in the phase shifter if the phases of the signal outputted from the DC SQUID and the reference signal are different. Another problem is that an output signal can not be obtained which is stable against the phase drift of the signal to be transmitted from the DC SQUID.